https://github.com/clojure/data.priority-map/
A priority map is similar to a sorted map, but sorts the entries by the
values rather than the keys in the map. Think of it as a kind of priority
queue with a full map-like API to query, add, adjust, and remove items and
their priorities.
The new 0.0.3 version addresses a common need where the items map to
complex values which contain the priority, or from which the priority can
be derived.
For example, consider a map:
{:apple {:weight 2, :color :red}, :orange {:weight 2, color :orange},
:banana {:weight 3, color :yellow}}
where you want to sort the map by the fruit's weight.
A common pitfall was to try to solve this with a custom comparator which
compares only on weight. This doesn't work properly because of the
Clojure/Java rule that valid comparators must be "total orders". Among
other things, this means the comparator can't have "ties" between unequal
objects (known as the trichotomy property).
[See https://groups.google.com/forum/#!msg/clojure/VKvH_eg_FtE/Kpy18zGtio4Jfor
one such discussion about this pitfall]
Users were instructed to rework their comparator into a total order, but
for some use cases, this can be a tricky proposition.
To address this need, the new version allows one to specify a "keyfn" which
extracts or computes the sort keys (i.e., priority) from the map's values.
This is done with one of two new constructors:
priority-map-keyfn (takes a custom keyfn)
priority-map-keyfn-by (takes a custom keyfn and custom comparator)
So for example, the above map could be expressed as:
(priority-map-keyfn :weight
:apple {:weight 2, :color :red}, :orange {:weight 2, color :orange},
:banana {:weight 3, color :yellow})
See the README for further explanation, and let me know if you have any
questions.
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